Ever since digital memories have been used for storing digital words, analog signal samples have been converted to digital signal samples and the digital words representing the samples have been stored in a digital memory. After optionally long storage times, the words representing the digital samples have been retrieved by reading from the memory. A digital-analog (D/A) converter is used to retrieve the corresponding analog signal samples.
This principle of storing a random number of analog signal samples, e.g. by using their individual addresses in the digital memory, is superior to the even older storage principle where the analog signal samples are each stored in individual sampling and holding circuits. This superiority mainly depends on the storage time of an analog holding circuit being limited if there is a demand that an analog signal sample shall be the same before and after storage.
With an operationally reliable digital memory, a digital word is always the same before and after storage, but the accuracy in storing an analog signal sample using a digital memory depends on how exactly the characteristic for analog-digital conversion used before storage agrees with the characteristic for digital-analog conversion used after storage. For good accuracy it is required that the D/A converter be tuned in exactly with the analog-digital conversion circuit. If satisfactory characteristic tuning and satisfactory compensation of the differences between the necessary amplifiers are technically possible at all, they will in any case be very expensive.